Abstract: Here we report the discovery of an extremely massive and large supercluster
(called Saraswati) found in the Stripe 82 region of SDSS. This supercluster is
a major concentration of galaxies and galaxy clusters, forming a wall-like
structure spanning at least 200 Mpc across at the redshift $z \approx 0.3$.
This enormous structure is surrounded by a network of galaxy filaments,
clusters, and large, $\sim40 - 170$ Mpc diameter, voids. The mean density
contrast $\delta$ (relative to the background matter density of the universe)
of Saraswati is $\gtrsim 1.62$ and the main body of the supercluster comprises
at least 43 massive galaxy clusters (mean $z = 0.28$) with a total mass of
$\sim 2 \times 10^{16} M_{\odot}$. The spherical collapse model suggests that
the central region of radius $\sim20$ Mpc and mass at least $ 4 \times 10^{15}
M_{\odot}$ may be collapsing. This places it among the few largest and most
massive superclusters known, comparable to the most massive `Shapley
Concentration' ($z \approx 0.046$) in the nearby universe. The Saraswati
supercluster and its environs reveal that some extreme large-scale, prominent
matter density enhancements had formed $\sim4$ Gy in the past when dark energy
had just started to dominate structure formation. This galactic concentration
sheds light on the role of dark energy and cosmological initial conditions in
supercluster formation, and tests the competing cosmological models.